Modern integrated circuits are composed of multiple layers of conductors, insulators, and semiconductors. Many modern integrated circuits are fabricated using “flip chip” technology in which the circuit is mounted upside-down onto a carrier. To inspect or alter interior layers of such circuits after the chip is mounted, it is necessary to approach the circuit from the backside. Semiconductor wafers are typically several hundred microns thick, so it is necessary to remove a substantial amount of material from the back side of the circuit before reaching the circuit. When accessing circuitry from the backside, there are no reference points for navigation, that is, there is no easy way to determine exactly where a particular feature on the circuit is located. Thus, to access the circuitry on a flip chip, one must determine where to remove material to expose the circuit from the back side and when to stop removing material to prevent damage to the circuit. Determining when to stop milling is referred to as “end pointing.”
Removing the backside material is typically performed in several steps. A first step typically includes a process, such as chemical mechanical polishing, that rapidly thins the entire chip, leaving sufficient material to provide mechanical strength for handling the chip. A subsequent step involves making a large hole in the material centered on the estimated position of the circuit feature of interest. Such a process is typically done using a laser or an ion beam. A process that rapidly removes material is typically not capable of stopping at a precise depth, so as the back side hole approaches the circuit, a second, more accurate process is typically used.
One method of slowing approaching the circuit from the backside uses ion beam milling along with an “end-pointing” technique that indicates when the feature to be exposed is near or is reached. In one end-pointing technique, a light is shown into the hole, and the light induces a current as the hole approaches a transistor region of the circuit. As the optical beam-induced current increases, the user knows that he is getting closer to the transistor region of the circuit.
Another endpointing technique, described in U.S. Published Pat. App. No. 2002/0074494 to Lundquist, uses focused ion beam milling to approach an active transistor region of the circuit from the backside. As the ion beam approaches the circuit, it creates charge carriers that cause a leakage current through the transistor. The ion beam is modulated, and a frequency sensitive amplifier amplifies the power supply leakage current at the modulation frequency. When the current achieves a certain level, the user assumes that the ion beam is very close to the active transistor region of the circuit. While this method can tell when a user is getting close to an active transistor region, it does not provide information about where on the surface the ion beam is impacting, other than that it is impacting near an active transistor region.
One common technique for determining when to stop milling, whether on the back side of a flip chip or on the front side of a conventional circuit, is to observe an image of the circuit when a layer has been milled through. Although an optical microscope can be used to form an image, the resolution of an optical microscope is on the order of 0.5 μm, which is insufficient to observe to circuit features, which can be on the order of 0.1 μm. A more appropriate method of observing microscopic devices is by using charged particle beam imaging, such as scanning ion microscopy or scanning electron microscopy.
A charged particle beam, such as a focused ion beam or an electron beam, is scanned across a surface. The impact of the charged particle beam causes the ejection of various particles, including secondary electrons, backscattered electrons, and ions. The number of particles emitted from each point depends on the composition and the topography at the point. An image is formed on a video monitor, with the brightness of each point on the image corresponding to the number of particles emitted from the surface at a corresponding point. An image can provide information to navigate by, if the image can be correlated to known information about the circuit.
The work piece typically is typically supported on a stage. The stage can move in three dimensions, “X,” “Y,” and “Z,” and movement of the stage and beam is specified and controlled using system coordinates. A work piece typically has its own coordinate system used by its designers to specify where various features are formed. By finding registration marks, known as “fiducials,” that are incorporated into the work piece, it is possible to correlate work piece coordinates with system coordinates, so that a user can specify a position on the work piece using work piece coordinate, and the system can move the stage and direct the beam, that is “navigate,” to that location. Such correlation is referred to as registration. While milling a chip from the back side, the fiducials are not visible and so it is difficult to register the work piece and to find a desired location.
While imaging techniques are useful for navigating in a plane, such techniques have disadvantages for end pointing. When using imaging to determine when a layer is exposed, the layer can be damaged before the endpoint is determined. Moreover, in order to find reference points in an image to determine where on the circuit the beam is located, it would be necessary to expose by trial and error and relatively large area, potentially damaging each area that is exposed.
U.S. Pat. No. 6,548,810 to Zaluzec for a “Scanning Confocal Electron Microscope” teaches an electron microscope that can image subsurface features, but because the system uses transmitted electrons, the substrate must be relatively thin and the system configured for detecting transmitted electrons, and so cannot be readily used in existing SEMs.